H. f. heating apparatus

ABSTRACT

A high frequency dielectric heating apparatus with a novel stray-field platen. The platen comprises a first group of a parallel rod electrodes spaced apart in a first plane. A second group of parallel rod electrodes are alternately arranged between and in parallel with the first group of electrodes. In one region of the platen the rods of the second group are displaced in a direction away from the material to be heated, whereas in a second region of the platen the rods of the second group are closer to the material, or even in the same plane as the rods of the first group. The two regions produce different heating effects on the dielectric material. A high frequency generator is connected across adjacent electrodes. The spacing between the electrodes can be varied and the electrodes can be inclined at an angle to the lateral direction of the material to provide further variations in the heating effect across the platen.

United States Patent 4 [151 3,701,875

. Witsey et al. g I I [451 Oct. 31, 1972 [54] H. F. HEATING APPARATUS2,871,332 1 1/1969 Northmore et al. ..219/10.8i [72] lnvemm' 3:3 :22,323 221: 3,082,710 3/1963 Holland ..219/10.s1 x

C P y i d S w 15 Rothstein. X .both of a- Primary Examiner-J. V. Truhe I[73] Assignee; lnterhern Limited Assistant ExaminerI-l ugh D. JaegerFiled: June Attorney-Frank R- Tflfal'l [21] Appi. No.2 45,994 [57]ABSTRACT A high frequency dielectric heating apparatus with a [30]Foreign Application Priority Data novel stray-field platen. The platencomprises a first group of a parallel rod electrodes spaced apartin aJune 30, 1969 Great Britain ..32,928/69 first plane. A Second group ofparallel rod elgctmdes are alternately arranged between and in parallelwith of In of platen the rods of the second group are displaced in a[58] Field of Search...2l9/ 10.81, 10.69, 10.61, 10.71 direction awayfrom the material to be heated,

whereas in a second region of the platen the rods of [56] References thesecond group are closer to the material, or even in v UNITED. STATES Pthesame P1338 as rods ,ofhthe first gtrfoup. The r3110 Y Y 7 .1 regionspro uce erent eating e ects on e 2,960,777 1l/1960 Doll 219/ l0.6l-'Xdielectric material but, frequency genera-OHS com 3,461,263 1 i969Manwaring "219/ 10.61 X. nected across adjacent electrodes; The spacing3,469,054 9/ 1969 Serota .219/10-61 between the electrodes. can b variedthe elec 3,364,294 H1968 Gambian et al. ....'....2l9/10.61 trodes can beinclined at an angle to the lateral 2,473,251 6/1949 HS ..219/10.61direction of the material to provide further variations 2 in the heatingeffect across the platen. 2,640,141 5/1953 Yore et al. ..2l9/ 10.81 X 14Claims, 4 Drawing Figures PATENTED UN 3 1 1912 saw 1 or 2 jliiim w i Imm 31 m SHEET 2 OF 2 PM JK/ 4 Fig.4

INVENTORS NOEL J. c. WITSEY JAMEIS c. CLARK By AGEIi having an airdielectric. When a material to be heated is brought near to-the platen,the field between the rods will divert to pass through the material andthe amount of the field so diverted will depend on the dielectricconstant of the material. If the material also has a high dielectricloss factor, a large amount of energy will be absorbed from thehigh-frequency field generated between the two electrode systems, thatis the rods of the platen, as a result of the rapidly alternatingvoltage difference between the rods, and dielectric heating will occurin the material.

Water is a dielectric having a high dielectric constant together with ahigh dielectric loss factor and this property can be utilized incontinuous flow production .techniquesfor the drying of materials suchas, for example, leather, paper and carpets. Thus, by passing the wetmaterial close to the surface of a stray field platen the water willabsorb energy from the high frequency 2 reduce the heating effect of thehigh frequency field produced by theplaten when energized by the supplyin comparison with the heating effect produced in the absence of saiddisplacement. When the material to be heated has a high moisturecontent, the predetermined region can be that part of the platenadjacent to which the material to be heated is first passed so thatexcessively rapid heating and drying of the material can be prevented atthe point where the water content and hence the dielectric loss isgreatest. Alternatively, when heating a material exhibiting an increasein the dielectric loss factor as a function of a rise in temperature,

the predetermined region can be that part of the platen adjacent towhich the material passes at a later stage of the dielectric heatingprocess. The amount of the displacement applied to the alternate rodscan be progressively changed in the direction of travel of the materialto be heated. In addition, the relative spacing of the electrodes can bechanged from one region to another to modify the relative heatingeffect. The rod electrodes can be inclinedto the direction of travel ofthe material to be heated at an angle other than 90.

field and evaporation will be assisted. In such a process the heatingeffect will occur selectivelyin the damp regions of the material sincethe dielectric loss factor of water is normally much greater than thedielectric loss of the base material. Thus as the wet material enterstheregion of the stray-field platen alarge amount of heating will takeplace and a, heavy electrical load will be presented to the platen intheinitial stage of the drying process. However, as the material is fedacross the platen and becomes drier, the amount of energy dissipatedwill be greatly reduced as the water content falls. Thus the electricalloading of the platen will be non-uniform and it will be difficult toadjust the highfrequency drive to the platen so that excessively rapidheating and drying of thematerial does not take place in the initialstages while ensuring at the same time that the material is properlydried towards the end of its passage over the platen.

H.F. heating apparatus employing a stray field platen can also beemployed for curing resin and P.V.C. formulations and it has been foundthat some of these formulations result in a material having a dielectricloss factor which increases with an increase in temperature. In thiscase it is found that a heavy electrical load is presented to the platennot in the entrance region of the platen but in a later region, whichmay be at the exit region of the platen. However, in this case too, asimilar difficulty will arise in the adjustment of the highfrequencydrive to the platen.

It is an object of the present invention to provide an improved formofhigh-frequency heating arrangement in which the above difficulties arealleviated.

. In accordance with the present invention, there is provided astray-field platen comprising a plurality of rod-like electrodesarranged in operation so that adjacent rods are connected across ahigh-frequency The material to be heated can be passed horizontallyabove or below the stray-field platen, or alternatively thestray-fieldplaten can be mounted vertically and the material fed past it in avertical direction. The electrode rods can be of curved form and canthus be arranged to present a radiused face to the material to beheated. In order to avoid the disadvantages resulting from theoccurrence of standing waves in the stray-field platen, high-frequencycurrent can be supplied to the rod-like electrodes in the manner setforth and claimed in our British Pat. Specification No. 1,017,289.

In order that the present invention may be clearly understood andreadily carried into effect, certain embodiments thereof will now bedescribed, byway of example, with reference to the accompanyingdrawings, in which: 1

FIG. 1 is a longitudinalisectional diagram illustrating a stray-fieldplaten employing the invention,

FIG. 2 is a detail illustrating a method of adjustment of theelectrodes, g

, FIG. .3 is a plan diagramillustrating a feature of the invention, and

no. '4 is a plan diagram illustrating a further feature of theinvention.

Referring to FIG. 1 which illustrates schematically in longitudinalvertical section one arrangement of the electrodes of a stray-fieldplaten embodying the invention and which is-adapted for drying materialhaving a high water content. A web 1 of material in which the watercontent is to be reduced, such as, for example,

carpeting coated on the back with a water based emulsion of latex orP.V.C. or a pulp mat or paper, is passed over rollers 2 made of alow-loss dielectric material and beneath a stray-field platen formed byan array of parallel rod electrodes 3 and 4 arranged to lie across j thewidth of the web 1. The electrodes 3 are arranged at a substantiallyconstant height above the web 1.

Each of the electrodes 4 is arranged at a position between andequidistant from the two adjacent elec-. trodes 3, and at that end ofthe stray-field platen first reached by the web 1 each electrode 4 israised above the level of the adjacent electrodes 3 to an extentdependent on the reduction in heating eflect desired over travel of theweb, may give rise to standing waves along the platen. To overcome thisproblem, it is preferable to divide the platen up into groups ofelectrodes and employ the feeding arrangement described and claimed inour British Pat. No. 1,017,289 and illustrated in FIG. 1 by the seriesvariable capacitor 7.

It will be seen that by raising the electrodes 4 above the level of theelectrodes 3 in the first part of the platen, the relative amount of thehigh-frequency field present between the electrodes 3 and 4 that canpass through the web 1 is reduced. Thus, where the web 1 contains thegreatest amount of moisture, the heating effect of the'field is reduced.

If this were not done the heating eflect would be too great, causing themoisture to be given off too rapidly and damaging the web. Alternativelythe very moist web could so load the first part of the platen that theoutput of the oscillator in the high-frequency power source would bereduced and the web would not be dried satisfactorily. Thus bydisplacing the alternate rod-like electrodes 4 in a direction away fromthe web 1 within the first part of the stray-field platen, thesedifficulties can be overcome in a simple and convenient way.

The displacement of the electrodes 4 away from the web 1 over apredetermined region of the stray-field platen can be adjusted orpre-set. One method of supporting the electrodes 4 so that theirdisplacement can be adjusted is illustrated in FIG. 2. The electrodes 4are mounted on brackets 10 having slots 11, 12 cut longitudinallytherein. The brackets 10 are bolted by bolts l3, 14 to a bus-bar 15comprising two spaced parallel conducting bars 16, 17. The bolts 13, 14pass through the slots l1, l2 in'the brackets 10, between the bars 16,17, through a clamping plate 18, to be retained by nuts, not shown, onthe far side of the plate 18. The electrodes 3 can be similarly attachedto a further bus-bar 20.

Alternatively the electrodes 4 can be mounted on an assembly which canbe raised or lowered while the apparatus is operating so thatadjustments can be made of the relative heating effect over differentparts of the platen while the heating is taking place. Such anadjustment can be carried out manually by means of a handwheel or lever,or it can be performed by an electric motor or by hydraulic orair-actuated means, and can be controlled automatically, for example, inresponse to measurements of the water content of the input web ofmaterial to be dried.

It will be understood that although a gradual change in the displacementof the electrodes 4"with distance along the stray-field platen is shownin FIG. 1, the displacement can alternatively be made-to change in oneor more steps until the electrodes 4 lie in the same plane as theelectrodes 3.

In addition to the displacement of the electrodes 4 away from the web tobe dried in the region in which a reduced heating effect is required, afurther reduction in heating effect can be brought about by increasingthe spacing between the electrodes is illustrated, with reference to theremoval of moisture from a web, in a plan view in FIG. 3 in which thespacing between the electrodes 3 and 4 is greater over the region wherethe web enters the stray-field platen and is decreased towards the exitend of the platen.

When a web having a high moisture content enters the platen there may bea tendency for too great a load to be placed across the electrodes atthe input end of the platen, and this may adversely effect the poweroutput of the high-frequency generator feeding the platen. The initialload can be further spread over the electrodes of the platen byinclining the electrodes 3, 4 to the lateral direction as illustrated inthe plan view of FIG. 4. In this way the increased load at the input endof the platen can be distributed over a number of electrode pairs 3, 4.

While the invention has been described with reference to an embodimentin which the stray-field platen is arranged above a horizontal web, thestrayfield platen can be placed underneath the web in some applicationsand alternatively both the web and the platen can be arranged in avertical or inclined plane if desired. The electrode rods 3 and 4 neednot be straight but can be curved, for example, they can be bowed in thedirection towards the web 1.

Furthermore, by employing the invention, the relative distribution ofthe dielectric heating effect can readily be controlled over anypredetermined region of a stray-field platen. Thus the invention canalso be employed in the curing of resin formulations in which thedielectric loss and therefore the heating effect increases during thecuring process and/or as a result of an increase in the temperature ofthe material. The embodiment illustrated in FIG. 1 can be employed forthis purpose by displacing the alternate electrodes 4 near the exit in adirection away from the platen, while bringing the electrodes 4 near theentrance towards the web '1. until they are at the same distance fromthe web 1 as the electrodes 3. Alternatively theweb 1 could be fed inthe opposite direction with respect to the original arrangement of theelectrodes 3 and 4. The invention can also be employed when a waterbased resin emulsion is to be cured such that it is desirable to reducethe heating effect both at the start and at the finish of the operation.In this case alternate electrodes would be displaced away from the webin the entrance and exit regions of the platen, but not over the centralregion.

What we claim is:

1. A stray-field platen for high-frequency heating apparatus comprisinga high frequency energy supply, a plurality of rod-like electrodesarranged in operation so that adjacent rod-like electrodes are connectedacross said high-frequency supply, one group of alternate electrodesbeing spaced apart so as to define a path of travel adjacent thereto forthe material to be heated and another group of alternate rod-likeelectrodes being alternately arranged on the same side of the materialand between the electrodes of said one group and displaced therefrom ina direction away from the material to be heated within a predeterminedregion of 'ment.

2. A stray-field platen as claimed in Claim 1 in which saidpredetermined region is that part of the platen adjacent to which, inoperation, the material to be heated is first passed.

3. A stray-field platen as claimed in claim 1 wherein the physicaldisplacement of the other group of alternate rod-like electrodes fromsaid travel path progressively varies along the platen in the directionof travel of the material to be heated.

4. A stray-field platen as claimed in claim 1 wherein the lateralspacing between said rod-like electrodes varies in the direction oftravel of the material to be heated.

5. A stray-field platen for high-frequency heating apparatus comprisinga high frequency energy supply, a plurality of rod-like electrodesarranged in operation so that adjacent rod-like electrodes are connectedacross said high-frequency supply, one group of alternate electrodesbeing spaced apart so as to define a path of travel adjacent thereto-forthe material to be heated and another group of alternate rod-likeelectrodes being alternately arranged between the electrodes of said onegroup and being displaced therefrom .in a direction away from thematerial to be heated within a predetermined region of said platen toreduce the heating effect on the material by the high-frequency fieldproduced by the platen over said region in comparison with the heatingeffect otherwise produced in the absence of said .displacemenL-saidrod-like electrodes being arranged .in the platen at an oblique angleother than 90 relative to the direction of travel of the materialto beheated.

6. A stray-field platen as claimed in claim 1 wherein said electrodesare divided into two groups, the electrodes of the first group beingarranged in parallel in a first plane across which the material to beheated will pass, and the electrodes of the second group beingalternately arranged between the electrodes of the first group in asecond plane and parallel to said first group of electrodes.

7. A stray-field platen as claimed in claim 6 wherein the spacingbetween adjacent electrodes in a direction orthogonal to thelongitudinal axes thereof of a first region of the platen is differentfrom the electrode spacing in a second region of the platen.

8. A stray-field platen as claimed in-claim 6 wherein the electrodes ofsaid first and second groups are arranged at a non-perpendicular anglerelative to the direction of travel of the material to be heated.

9. A stray-field platen for high-frequency heating apparatus comprisinga high frequency energy supply, a plurality of rod-like electrodesarranged in operation so that adjacent rod-like electrodes are connectedacross said high-frequency supply, one group of alternate electrodesbeing spaced apart so as to define a path of travel adjacent thereto forthe material to be heated and another group of alternate rod-likeelectrodes being alternately arranged between the electrodes of said onegroup and being displaced therefrom in a dire on awga fgorn the materialto be heated witgg a pre etermln gm of sai p aten to reduce the ingeffect on the material by the high-frequency field produced by theplaten over said region in comparison with the heating efiect otherwiseproduced in the absence of said displacement, and wherein the distancebetween given ones of said alternate rod-like electrodes and saidmaterial varies along a portion of the platen in the direction of travelof the material. A

10. A high frequency heating apparatus for a material to be heated as itis moved along a given path of travel comprising, a source of highfrequencyelectric energy, a stray-field platen assembly that includestwo groups of rod electrodes sequentially arranged adjacent the path oftravel of said material to be heated and on the same side thereof, theelectrodes of one group being spaced apart and equidistant from thematerial as it moves along said travel path, the electrodes of the othergroup being alternately located between the electrodes of said one groupwith at least some of theelectrodes of said other group positioned at agreater distance from said material than the electrodes of said onegroup, and means for connecting the electrodes to the terminals of saidenergy source whereby different regions of the platen in the directionof travel of the material produce difierent heating effects on thematerial adjacent thereto.

11. A heating apparatus as claimed in claim 10 wherein the electrodes ofsaid one group are mounted in parallel in a first plane and at leastsome of the electrodes of said other group are mounted in parallel in asecond plane and parallel to said one group of electrodes.

12. A heating apparatus as claimed in claim 11 wherein the lateralspacing between adjacent electrodes in a first region of the platen isdifierent than the lateral spacing between adjacent electrodes in asecond region of the platen.

13. A heating apparatus as claimed in claim 10 wherein said electrodesare arranged in parallel in the platen at an oblique angle relative tosaid path of travel of the material. I

14. A heating apparatus as claimed in claim 10 including means formounting other ones of the electrodes of said other group in positionsat a still greater distance from said material to be heated.

1. A stray-field platen for high-frequency heating apparatus comprisinga high frequency energy supply, a plurality of rodlike electrodesarranged in operation so that adjacent rod-like electrodes are connectedacross said high-frequency supply, one group of alternate electrodesbeing spaced apart so as to define a path of travel adjacent thereto forthe material to be heated and another group of alternate rod-likeelectrodes being alternately arranged on the same side of the materialand between the electrodes of said one group and displaced therefrom ina direction away from the material to be heated within a predeterminedregion of said platen to reduce the heating effect on the material bythe high-frequency field produced by the platen over said region incomparison with the heating effect otherwise produced in the absence ofsaid displacement.
 2. A stray-field platen as claimed in Claim 1 inwhich said predetermined region is that part of the platen adjacent towhich, in operation, the material to be heated is first passed.
 3. Astray-field platen as cLaimed in claim 1 wherein the physicaldisplacement of the other group of alternate rod-like electrodes fromsaid travel path progressively varies along the platen in the directionof travel of the material to be heated.
 4. A stray-field platen asclaimed in claim 1 wherein the lateral spacing between said rod-likeelectrodes varies in the direction of travel of the material to beheated.
 5. A stray-field platen for high-frequency heating apparatuscomprising a high frequency energy supply, a plurality of rod-likeelectrodes arranged in operation so that adjacent rod-like electrodesare connected across said high-frequency supply, one group of alternateelectrodes being spaced apart so as to define a path of travel adjacentthereto for the material to be heated and another group of alternaterod-like electrodes being alternately arranged between the electrodes ofsaid one group and being displaced therefrom in a direction away fromthe material to be heated within a predetermined region of said platento reduce the heating effect on the material by the high-frequency fieldproduced by the platen over said region in comparison with the heatingeffect otherwise produced in the absence of said displacement, saidrod-like electrodes being arranged in the platen at an oblique angleother than 90* relative to the direction of travel of the material to beheated.
 6. A stray-field platen as claimed in claim 1 wherein saidelectrodes are divided into two groups, the electrodes of the firstgroup being arranged in parallel in a first plane across which thematerial to be heated will pass, and the electrodes of the second groupbeing alternately arranged between the electrodes of the first group ina second plane and parallel to said first group of electrodes.
 7. Astray-field platen as claimed in claim 6 wherein the spacing betweenadjacent electrodes in a direction orthogonal to the longitudinal axesthereof of a first region of the platen is different from the electrodespacing in a second region of the platen.
 8. A stray-field platen asclaimed in claim 6 wherein the electrodes of said first and secondgroups are arranged at a non-perpendicular angle relative to thedirection of travel of the material to be heated.
 9. A stray-fieldplaten for high-frequency heating apparatus comprising a high frequencyenergy supply, a plurality of rod-like electrodes arranged in operationso that adjacent rod-like electrodes are connected across saidhigh-frequency supply, one group of alternate electrodes being spacedapart so as to define a path of travel adjacent thereto for the materialto be heated and another group of alternate rod-like electrodes beingalternately arranged between the electrodes of said one group and beingdisplaced therefrom in a direction away from the material to be heatedwithin a predetermined region of said platen to reduce the heatingeffect on the material by the high-frequency field produced by theplaten over said region in comparison with the heating effect otherwiseproduced in the absence of said displacement, and wherein the distancebetween given ones of said alternate rod-like electrodes and saidmaterial varies along a portion of the platen in the direction of travelof the material.
 10. A high frequency heating apparatus for a materialto be heated as it is moved along a given path of travel comprising, asource of high frequency electric energy, a stray-field platen assemblythat includes two groups of rod electrodes sequentially arrangedadjacent the path of travel of said material to be heated and on thesame side thereof, the electrodes of one group being spaced apart andequidistant from the material as it moves along said travel path, theelectrodes of the other group being alternately located between theelectrodes of said one group with at least some of the electrodes ofsaid other group positioned at a greater distance from said materialthan the electrodes of said one group, and means for connecting theelecTrodes to the terminals of said energy source whereby differentregions of the platen in the direction of travel of the material producedifferent heating effects on the material adjacent thereto.
 11. Aheating apparatus as claimed in claim 10 wherein the electrodes of saidone group are mounted in parallel in a first plane and at least some ofthe electrodes of said other group are mounted in parallel in a secondplane and parallel to said one group of electrodes.
 12. A heatingapparatus as claimed in claim 11 wherein the lateral spacing betweenadjacent electrodes in a first region of the platen is different thanthe lateral spacing between adjacent electrodes in a second region ofthe platen.
 13. A heating apparatus as claimed in claim 10 wherein saidelectrodes are arranged in parallel in the platen at an oblique anglerelative to said path of travel of the material.
 14. A heating apparatusas claimed in claim 10 including means for mounting other ones of theelectrodes of said other group in positions at a still greater distancefrom said material to be heated.